Owing to the promotion of the personal computer (PC), the backup power-supply device is becoming significant. Nowadays, the representative backup power-supply device is the uninterruptible power system (UPS). The UPS is usually designed to have AC power output for supplying power for a period of time when a power outage occurs. Hence, the UPS is able to sustain supplying the power to an electric appliance during the power outage so that the electric appliance can operate without interruption. The UPS is usually employed in a complicated and large-scale electric appliance that can not be interrupted during the power outage, for example, a workstation. Therefore, the cost of UPS must be relatively high.
Referring to FIG. 1, a power supply 13 of a PC 11 includes an electromagnetic interference filter 131, an AC/DC converter 132, a DC/DC converter 133, and a feedback control circuit 134. If a high-level operating system, such as Windows 95/98, Windows NT, UNIX and so forth, is running on a PC, some serious damages like data loss may occur during the power outage.
Therefore, a UPS is needed to cooperate with a PC for supplying a backup power during the power outage. However, the UPS at least includes an electromagnetic interference filter 121, an AC/DC converter 122, a DC/AC converter 123, a charger 124, and a rechargeable battery cell 125. The UPS is quite complicated and it is not practical for a PC because the users of a PC are mainly involved in the fields of application and entertainment, whereas the users of a workstation are mainly involved in the fields of research and development. Actually, a PC needs only three to five minutes for proceeding the data storage and shutdown processes during the power outage. Therefore, it is desirable to develop a backup power-supply device which can provide enough power for storing data and shutting down the PC.